1869.] 



Observations of the Sun. 



355 



is narrow and is not often single ; higher up, the stems, so to speak, inter- 

 twine, and the prominence expands and soars upward until it is lost in 

 delicate filaments, which are carried away in floating masses. 



Since last October, up to the time of trying the method of using the 

 open slit, I had obtained evidence of considerable changes in the pro- 

 minences from day to day. With the open slit it is at once evident that 

 changes on the small scale are continually going on ; it was only on the 

 14th inst. that I observed any change at all comparable in magnitude and 

 rapidity to those already observed by M. Janssen. 



About 9 h 45 m on that day, with a tangential slit I observed a fine dense 

 prominence near the sun's equator, on the eastern limb. I tried to sketch 

 it with the slit in this direction ; but its border was so full of detail, and the 

 atmospheric conditions were so unfavourable, that I gave up the attempt in 

 despair. I turned the instrument round 90° and narrowed the slit, and 

 my attention was at once taken by the F line ; a single look at it taught 

 me that an injection into the chromosphere and intense action were taking 

 place. These phenomena I will refer to subsequently. 



At 10 h 50 m , when the action was slackening, I opened the slit ; I saw at 

 once that the dense appearance had all disappeared, and cloud-like filaments 

 had taken its place. The first sketch, embracing an irregular prominence 

 with a long perfectly straight one, which I called A, was finished at 1 l h 5 m , 

 the height of the prominence being 1' 5", or about 27,000 miles. I left the 

 Observatory for a few minutes ; and on returning, at ll h 15 m , I was as- 

 tonished to find that part of the prominence A had entirely disappeared ; not 

 even the slightest rack appeared in its place : whether it was entirely dis- 

 sipated, or whether parts of it had been wafted towards the other part, I do 

 not know, although I think the latter explanation the more probable 

 one, as the other part had increased. 



We now come to the other attendant phenomena. First, as to the F 

 line. In my second paper, under the above title, I stated that the F 

 line widens as the sun is approached, and that sometimes the bright line 

 seems to extend on to the sun itself, sometimes on one side of the F line, 

 sometimes on the other. 



Dr. Frankland and myself have pointed out, as a result of a long series 

 of experiments, that the widening out is due to pressure, and apparently not 

 to temperature per se ; the F line near the vacuum-point is thin, and it 

 widens out on both sides (I do not say to the same extent) as the pressure 

 is increased. Now, in the absence of any disturbing cause, it would ap- 

 pear that when the wider line shows itself on the sun on one side of the 

 F line, it should at the same time show itself on the other ; this, however, 

 it does not always do. I have now additional evidence to adduce on this 

 point, and this time in the prominence line itself, off the sun. In the pro- 

 minence to which I have referred, the F bright line underwent the most 

 strange contortion^, as if there were some disturbing cause which varied the 

 refrangibility of the hydrogen-line under certain conditions and pressures. 



